Gerotor pumps of the type having an inner rotor with a given number of lobes and an outer rotor with one additional lobe, are well known and include rotor assemblies of, without limitation, trochoidal, cycloidal, duo IC, duocentric, parachoid and other designs. Gerotor pumps are used in a variety of applications, such as engine and transmission oil pumps, and electrically driven gasoline pumps for automobiles. While gerotor pumps are widely used and provide good price/performance characteristics, in many applications, such as in oil pumps for internal combustion engines, gerotor pumps do suffer from a disadvantage in that it is not easy to alter their volumetric capacity. Accordingly, to obtain an equilibrium operating pressure in such applications, gerotor pump systems. typically have a pressure relief valve to limit the pressure of the working fluid supplied from the pump.
While such pressure relief valves do allow gerotor pump systems to achieve an equilibrium pressure, the volumetric capacity of the pump is not changed and thus the energy consumed by the pump continues to increase with the pump operating speed even after the equilibrium pressure is reached. Thus, energy from the engine is wasted when the pressure relief valve is diverting excess flow produced by the pump.
Published PCT Patent application WO 2004/057191 to Schneider teaches a variable volume gerotor pump wherein a rotatable adjusting ring has the outer rotor of the pump rotor assembly eccentrically mounted therein. By rotating the adjustment ring relative to the inlet and outlet ports, the volumetric capacity of the pump can be changed. While the Schneider reference does teach a variable volumetric capacity gerotor pump, the Schneider mechanism is complex, requiring a large number of parts, thus increasing the cost of the pump, and the pump is quite large in its radial dimensions which precludes its use in many circumstances.
Another variable volume gerotor pump is taught in U.S. Pat. No. 4,887,956 to Child, and in this pump, the inner rotor meshes with an axially adjacent pair of outer rotors. By altering the alignment of the two outer rotors, the volumetric capacity of the pump can be altered.
Published PCT Application WO 93/21443 to Hodge teaches another variable volume gerotor pump somewhat converse to the pump taught by Child. In the Hodge pump, two axially adjacent inner rotors turn in a single outer rotor. The volumetric capacity of the pump is altered by changing the alignment of the two inner rotors.
While Child and Hodge do teach variable capacity gerotor pumps, the resulting pumps are quite complex, as are the control mechanisms to vary the capacity. Further, the torque on the control shaft of each pump can be non-linear relative to the rotation angle, making it difficult to establish an equilibrium operating pressure.
U.S. Pat. No. 2,484,789 to Hill and subsequent similar patents provide various designs for a variable capacity gerotor pump where the inner rotor moves axially relative to the outer rotor, or vice versa, the volumetric capacity being dependent on the amount of overlap between the two rotors. A major disadvantage of these designs is that the sealing plates at each end of the rotor pair are shaped to mesh inversely with the rotor teeth and they rotate with the rotors. The pump inlet and outlet flows must therefore be fed to and from the rotors using a complex route such as a series of holes in one of the sealing plates and a distributor system, or radial holes in the outer rotor. Any such method is likely to restrict the inlet flow and lead to early onset of cavitation, which is probably one reason why such pump designs are not in common usage.